This invention relates to a fixture for holding a workpiece for machining, and more particularly to a fixture for holding and centering a hollow generally cylindrical or cup shaped workpiece for machining.
Workpieces are typically machined by pressing a cutting tool against the piece to remove portions of the workpiece until the desired dimensions are achieved. A known technique for machining includes holding the workpiece in a fixture and moving the cutting tool relative to the workpiece. The workpiece may be held stationary or it may be moved. It is desirable to machine hollow, generally cylindrical workpieces including brake drums and similarly shaped pieces. The workpiece will hereinafter be referred to as a brake drum, although any suitably shaped workpiece may be used.
The brake drum includes a hollow generally cylindrical body portion defining a longitudinal axis and having radially inner and outer surfaces, and an end portion or mounting face extending radially inwards from one end of the body portion. The brake drums are typically gripped on the radially inner or outer surfaces of the body portion by a plurality of grippers or chucks when the drum is machined. However, gripping the drum in this manner with sufficient pressure to hold it steady during machining tends to distort the cylindrical shape of the drum. Although the drum may have the desired dimensions after machining while it is still held in the fixture, it will often change its shape after the fixture gripping forces are removed. The shape of the finished brake drum may change so much after removing it from the fixture that it no longer has the desired dimensions. Furthermore, additional distortion may occur when the placement and direction of forces against the drum are changed as the cutting tool is moved during machining.
To accommodate the changing forces applied to the brake drum during machining, one prior art fixture device includes a plurality of discrete flexible chucking fingers actuated by a pliable inflatable ring for radially deflecting the fingers into gripping, non-distorting engagement with the body of the drum. Others have used a first plurality of lock-up fingers, and a second plurality of gripping fingers. The lock-up fingers are individually biased radially outward into engagement with the body of the brake drum to initially support the brake drum in a predetermined position, irrespective of the eccentricity of the body wall. The locking fingers locate the brake drum along an axis of the fixture. A bladder flexes the gripping fingers into engagement with the body to support the body for machining. However, the chucks grip the body along a single plane which does not prevent distortions by forces changing in the axial direction.
Another prior art fixture attempts to prevent distortions to a hollow, cylindrical workpiece using a relatively thin wall sleeve together with a sleeve of pliant material to grip the workpiece. The pliant sleeve conforms to the surface of the workpiece and exerts a uniform pressure on the workpiece. However, the forces exerted to hold the workpiece are not always sufficient to hold the workpiece stationary during machining.
It is desirable to hold the brake drum which sufficient pressure to keep the drum stationary during machining without distorting the drum. It is also desirable to locate the drum on the fixture in a manner such that the axis of the drum coaxially aligns with the axis of the fixture.